This invention relates to an improved cracking catalyst. More particularly, this invention relates to a zeolite-containing cracking catalyst which is improved by treatment with an aqueous solution containing a fluorine compound.
In the art of conversion of hydrocarbons, catalytic activity is usually described as either hydrogenative or acidic. Cracking reactions depend on the acidic activity of catalysts. Crystalline alumino-silicates, known generally as zeolites or molecular sieves, have proved exceptionally valuable as acidic and, hence, as cracking catalysts.
Cracking reactions are used to convert heavier hydrocarbons into products having a lower average molecular weight useful in the production of motor fuels. Cracked products generally have a lower octane number than that desired for gasoline blending components. To minimize further processing, such as catalytic reforming operations, it is desirable that the gasoline components of the products of the cracking operation have as high an octane number as possible.
It is known in the art that catalytic activity of solid contact catalysts may be enhanced by incorporating into the catalyst acid activators, such as fluorine. See generally Choudhary, Ind. Eng. Chem., Prod. Res. Dev., 16, 12 (1977). Prior to the advent of zeolite cracking catalysts, amorphous-type catalysts, for example, alumina and silica-alumina catalysts, were enhanced by treatment with various flourine compounds under anhydrous or aqueous conditions. For example, U.S. Pat. Nos. 2,336,165 (Connolly), 2,483,131 (Garrison), 2,694,674 (Starr et al.), and 2,848,380 (Thomas) all describe the preparation of improved alumina and silica-alumina cracking catalysts by treatment with various fluorine compounds.
More recent patents relate to the methods for fluoriding zeolite-containing catalyts. U.S. Pat. No. 3,137,656 (Mason et al.) describes a method for haliding a hydrocracking catalyst which may comprise crystalline alumino-silicates by treatment with an anhydrous halide. The resulting catalyst has enhanced hydrocracking activity but must be used in a carefully controlled anhydrous atmosphere to prevent stripping off of the loosely held halide. Use in a wet atmosphere or treatment with an aqueous halide agent is disclosed as detrimental to catalyst activity. U.S. Pat. No. 3,702,312 (Wilson) describes a method for incorporating fluorine into the crystal lattice of an alumino-silicate by a sequence of steps involving: removing sodium from the zeolite by ion exchange with a divalent alkaline earth metal, especially calcium; treating with a sequestering agent; contacting the exchanged zeolite with an aqueous solution containing aluminum, fluoride, and chloride ions; and washing with water. This patent demonstrates (Example I) the inapplicability of conventional halide impregnation techniques, such as treatment with aqueous hydrogen fluoride, for treatment of zeolites.
U.S. Pat. Nos. 4,097,368 (Hayes) and 4,098,679 (Hayes) describe dual-function composite catalysts comprising a combination of three or four metals on a porous carrier which may comprise a zeolite. Halogen may be added to the carrier in any suitable manner, for example, by treatment with an aqueous solution of hydrogen fluoride.
The present invention relates to a fluorided cracking catalyst comprising a zeolite in a silica-alumina matrix and to a process employing said catalyst. According to the present invention, a zeolite-containing cracking catalyst is fluorided by treatment with an aqueous solution containing a fluorine compound. According to one embodiment, the fluorided catalyst may be used per se as an improved cracking catalyst. In another embodiment, the fluorided catalyst is physically mixed with a metallic reforming catalyst and the composite is used as an improved cracking catalyst. Use of the catalysts of the invention in cracking reactions results in greater cracking activity and higher octane products than can be obtained with unfluorided cracking catalysts.